PurchaseOrderDetails: show related request literature

[Pineirin]

Show the related request literature if any in the purchase order details page. closes https://github.com/CERNDocumentServer/cds-ils/issues/568 New purchase order with related literature request. New purchase order without related literature request. New purchase order with error fetching related literature request.

Old purchase order with related literature request. Old purchase order without related literature request.

[Pineirin]

Additional feedback on the demo.

• Make the link open a new tab.
• Put the link as part of the "Order Information".